Proline cis/trans-Isomerase Pin1 Regulates Peroxisome Proliferator-activated Receptor �� Activity through the Direct Binding to the Activation Function-1 Domain

The important roles of a nuclear receptor peroxisome proliferator-activated receptor γ (PPARγ) are widely accepted in various biological processes as well as metabolic diseases. Despite the worldwide quest for pharmaceutical manipulation of PPARγ activity through the ligand-binding domain, very little information about the activation mechanism of the N-terminal activation function-1 (AF-1) domain. Here, we demonstrate the molecular and structural basis of the phosphorylation-dependent regulation of PPARγ activity by a peptidyl-prolyl isomerase, Pin1. Pin1 interacts with the phosphorylated AF-1 domain, thereby inhibiting the polyubiquitination of PPARγ. The interaction and inhibition are dependent upon the WW domain of Pin1 but are independent of peptidyl-prolyl cis/trans-isomerase activity. Gene knockdown experiments revealed that Pin1 inhibits the PPARγ-dependent gene expression in THP-1 macrophage-like cells. Thus, our results suggest that Pin1 regulates macrophage function through the direct binding to the phosphorylated AF-1 domain of PPARγ.     

Structural Basis for the Binding Specificity of Human Recepteur d'Origine Nantais (RON) Receptor Tyrosine Kinase to Macrophage-stimulating Protein

Recepteur d''origine nantais (RON) receptor tyrosine kinase and its ligand, serum macrophage-stimulating protein (MSP), play important roles in inflammation, cell growth, migration, and epithelial to mesenchymal transition during tumor development. The binding of mature MSPαβ (disulfide-linked α- and β-chains) to RON ectodomain modulates receptor dimerization, followed by autophosphorylation of tyrosines in the cytoplasmic receptor kinase domains. Receptor recognition is mediated by binding of MSP β-chain (MSPβ) to the RON Sema. Here we report the structure of RON Sema-PSI-IPT₁ (SPI₁) domains in complex with MSPβ at 3.0 Å resolution. The MSPβ serine protease-like β-barrel uses the degenerate serine protease active site to recognize blades 2, 3, and 4 of the β-propeller fold of RON Sema. Despite the sequence homology between RON and MET receptor tyrosine kinase and between MSP and hepatocyte growth factor, it is well established that there is no cross-reactivity between the two receptor-ligand systems. Comparison of the structure of RON SPI₁ in complex with MSPβ and that of MET receptor tyrosine kinase Sema-PSI in complex with hepatocyte growth factor β-chain reveals the receptor-ligand selectivity determinants. Analytical ultracentrifugation studies of the SPI₁-MSPβ interaction confirm the formation of a 1:1 complex. SPI₁ and MSPαβ also associate primarily as a 1:1 complex with a binding affinity similar to that of SPI₁-MSPβ. In addition, the SPI₁-MSPαβ ultracentrifuge studies reveal a low abundance 2:2 complex with ∼10-fold lower binding affinity compared with the 1:1 species. These results support the hypothesis that the α-chain of MSPαβ mediates RON dimerization.